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Micro-inverters vs. string inverters for solar PV

Enecsys launched its solar photovoltaic (PV) micro-inverters at Intersolar Europe in Munich on 9-11 June 2010. Assistant Editor Kari Larsen spoke to CEO Paul Engle about the advantages and disadvantages of micro-inverters compared with string inverters.

Enecsys says its solar PV micro-inverters have improved energy harvest “and therefore cut the cost of harvested power by up to 20% over the lifetime of the installation compared to a conventional system using string inverters.”

Unlike other micro-inverters, Enecsys uses thin-film capacitors instead of electrolytic capacitors. This enables the inverters to achieve a 25-year operating life, matching that of solar PV modules. It also means that they operate over a temperature range of -40°C to +85°C, and achieve peak efficiency of 94.1% within this temperature range.

“What we try to do, which is fundamentally different, is to make each panel independent.”

- Engle

Engle explains that normally, solar panels are connected in series that lead to a string inverter at the end of the circuit. However, these will “only work as well as the weakest one,” he says. If there is even the tiniest of shading on one panel, this will impact the whole output of a series-connected system.

On large solar plants, this is not really a problem as the panels are cleaned regularly and they have been constructed and designed to avoid any shading. Solar fields are also usually away from tall building and other obstructions.

“But on a residential or commercial roof top, you can have a lot of ways there can be shade,” Engle explains.

Safety in independence

“What we try to do, which is fundamentally different, is to make each panel independent.” Engle adds that: “It’s a very simple plug and play system.”

One important benefit of having an inverter per panel is that when you series connect panels, the total voltage increases for every panel added. “This string ends up typically carrying about 700-1600 V, which means it is really dangerous,” Engle says. It is enough to seriously electrocute someone.

“If you have a fire in your building, you could shut off the AC, but these things [solar panels] are sitting in the sun – if it’s daylight – and they are trying to produce power even though this thing has been disconnected. If you get a fireman spraying water on that roof and he is on a fire truck or a ladder, he would get electrocuted.”

If each panel has an inverter, however, this would not happen as the voltage would be too low to cause any harm.

Independence in more than one way

With micro-inverters, it is also possible to add panels later on – this cannot be done with string inverters as the inverter is customised to the total power output of the installation.

The micro-inverters also enable the use of panels from different producers and of different sizes within an installation.

“Even the panels themselves age differently, so after five or 10 years they may have 5% less output, and the string system will only work as well as the lowest one,” Engle says.

A monitoring system also come with the inverters where each solar panel can be monitored individually – unlike string inverters that can only monitor the system as a whole.

“If you have one of these [string inverters] on your house, the only way you will notice a problem, is you might notice that it’s a perfectly clear day today and yesterday, but yesterday you produced 5 kWh and today only 3 kWh.

“With our system it tells you exactly where the problem is,” Engle says.

Normally, specially trained electricians are needed for installing rooftop solar systems due to the danger of DC arcing, etc. With micro-inverters at each panel, only a standard electrician is needed to connect the wires to the electricity box.

"... you do it once and you are done!"

- Engle

“All you do is put the cable connectors together – if you can plug in cables and anyone can do that – you just need the roofers to mount the stuff on the roof and then very simply mount the inverters on the roof, and you hook the cables together and cables go down in a pipe into your electricity box in the house, and [you need] a standard electrician.”

Electrolytic vs. thin-film capacitors

String inverters usually use electrolytic capacitors which have high capacity storage, but Engle says they typically have short life spans, something that is reduced even further with fluctuating temperatures of outdoor installations.

However, electrolytic capacitors do have a unique inherent capability to store a lot of energy in a small space. “It’s a cost effective, really compact way of storing a lot of energy,” Engle says.

String inverters are typically warranted for five years. Some of the larger producers will sell warranties for up to 20-25 years, but “that warranty costs as much as the inverters. So you pay double or more,” Engle says.

Enecsys’ inverters are warranted for 25 years without any extra charge, and they are likely to last 35-40 years, “so you do it once and you are done!”

Enecsys uses thin-film capacitors in its inverters as they can do more or less the same things as electrolytic capacitors.

“But, I would add the advantage that thin-film capacitors are more reliable as they don’t depend on the same technology as electrolytics. The disadvantage is that they cannot hold much energy. What we have done, is that we have invented a means where we don’t need to store much energy. ... We have created a way that is patented that doesn’t require such storage charge,” Engle explains.

Why not use micro-inverters in large installations?

Engle says that with larger installations such as utility-scale solar parks, “it’s probably not the most cost effective way. They [the developers] tend to plan them so perfectly to minimise disadvantage, and the cost is a bit more for [micro-inverters] than if you have one huge box.

“The efficiency is not quite as high under perfect conditions, but if you have compromising conditions, we make a lot more energy than you do with a conventional system.”

In a test currently running at Enecsys’ facilities in Cambridge, UK, the experience is that when there is shading, the micro-inverter systems have up to 20% advantage in power output compared to string inverters.

The built environment – a niche market

“We are going to focus on residential and commercial roof tops. We will probably never do ground mounted field installations that are MW size,” Engle says, before adding: “The advantage about building on roofs is that roof tops are where people use electricity and you already have the grid there.

“In a lot of urban environments where there are lots of buildings around and perhaps a lot of tall buildings, you can still use those rooftops for very effective solar harvest,” Engle concludes.

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Comments

Efried said

14 July 2010
If I operate below 70V introducing 230V on my roof via micro-inverters is creating more regulative problems.What is really needed is cheap in-string MPPT and storage type inverters having some leveling out of grid voltage minima earning additional money ;-)

DI Zastor said

29 June 2010
Are these available to the market or just under test? What is the unit price?

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